1. Field of the Invention
The present invention is related to the field of particle classification and, in an important embodiment, to an air classifier system for simultaneously separating a single sand stream into two or more distinct grades of foundry quality sand.
2. Description of the Related Art
Particle separation and classification is a necessary part of many industrial processes. Air classification is effective in many instances and, through the introduction of a stream of particulate matter to an air stream, relies upon terminal velocity of the particles to separate particles of different sizes, shapes and composition. Particles remain in the air stream over a distance which is inversely proportional to their terminal velocities. Through the use of receiver sections located beneath the air flow, particles having similar velocities may be collected in respective sections.
Central to an air classification system is the flow of air through the classifier. Prior art methods of air classification typically rely on a blower or fan feeding air into the classification section, or the air going into this section comes from the recycle of air from such a fan or blower. Using such techniques, the air is extremely disturbed, with high levels of turbulence and severe swirling on a large scale. This turbulence and swirling reduces the accuracy of the classification achieved.
To deal with these severe swirl problems, prior art solutions have relied upon a screen section, including one or multiple screens, followed by a honeycomb arrangement to direct and smooth the air flow. U.S. Pat. No. 5,032,256 to Vickery is representative of such an air classifier system. U.S. Pat. No. 4,213,852 to Etkin also illustrates such a system, using multiple screens. With the prior art systems, swirl is reduced but remains a problem. In addition, while specifying the use of a honeycomb to remove large scale swirl from the incoming air, the patent to Vickery teaches that the cell length to cell diameter ratio (L/D) of the individual honeycomb cells should be 20/1, but in all cases must be higher than 8/1.
Prior art air classifier systems also suffer from ineffective control of the particle feed stream. Using prior art techniques, the feed stream of particles entering an air classifier often falls as a thin stream transverse to the flowing air. This has an adverse effect on the operation of the classifier, except at very low feed rates, e.g., less than 10 gm/min/cm of the transverse feed stream (or approximately 1 kg/min for a classifier one meter in width). At higher, and more practical, feed rates, the particle concentration is so high that the particles do not fall individually as they enter the classifier, but as a solid xe2x80x9ccurtainxe2x80x9d. This has two deleterious effects. First, the incoming feed curtain blocks the air flow at the top of the classifier, diverting the air downward, negating the effort of creating an even, undisturbed air stream. The result is the particles are not separated nearly as well as would be anticipated were they to be acted upon individually by the air. Second, the particles falling in the feed curtain are not separated during the initial part of their fall into the air stream. The fine particles fall along with the larger particles, instead of being blown free of them. This results in a defective separation, with smaller particles falling into earlier receiving chambers meant for the large particles. These effects become more pronounced as the feed rate increases.
Accordingly, a need exists for an air classifier system having adequate control of both swirl and the incoming particle feed stream in order to obtain particle separations of discrete ranges having greater internal uniformity.
In view of the foregoing, one object of the present invention is to reduce swirl in the air flow to an air classifier system through the use of quiescent ambient air which is pulled from outside the classifier rather than pushed into the classifier.
Another object of the invention is to provide an air classifier that permits use of a honeycomb with a cell length to cell diameter ratio (L/D) as low as 4.
A further object of the invention is to provide an air classifier in which the honeycomb is placed before the screen section in order to achieve maximum reduction in swirling of the air.
A still further object of the invention is to provide an air classifier having a screen section which includes only two or three screens and yet reduces mean variations in velocity measured at evenly spaced positions across the airstream to less than 5% of the mean velocity.
An additional object of the invention is an air classifier in which the incoming feed stream is spread by widening the aperture through which feed enters the classifier and directing the feed stream through one or more vibrating screens.
Yet another object of the invention is to provide an air classifier with enhanced particle separation capability even at high feed rates through the introduction of an upward air flow within the receiving chambers.
In accordance with these and other objects, the present invention is directed to an air classifier for separating particulate material. The air classifier includes a horizontally disposed classification chamber having an upstream end and a downstream end. The upstream and downstream ends allow air to flow into and out of the chamber, respectively. An air suction device is located adjacent the downstream end of the chamber for drawing air through the chamber from the upstream end to create a chamber air stream. Particulate matter is fed into the chamber through a feed stream input located in an upper part of the chamber proximate the upstream end. Particles entering the chamber are entrained in the chamber air stream.
The air classifier further includes a screen section situated adjacent to and upstream of the upstream end of the chamber, and a honeycomb located adjacent to and upstream of the screen section. Air entering the chamber first passes through the honeycomb, and then through the screen section. The honeycomb takes out the swirl in the air and the screen section slows down the faster moving portions of the air more than the slower moving portions. As a result, the velocity profile of the smoothed air is much more constant across the entire flow path. Particles introduced to the chamber through the feed stream input are entrained in the smoothed air as it exits the screen section.
A plurality of receiver sections are serially disposed in an upstream to downstream arrangement along the bottom of the chamber. As particles entrained in the chamber air stream fall out, these particles are collected in the receiver sections. Larger and/or heavier particles fall out sooner and are collected in receiver sections nearest the feed stream input, while smaller/lighter particles remain entrained for a longer period and are collected in receiver sections closer to the downstream end of the chamber.
In a preferred embodiment, the feed stream input includes a vibrating screen feeder which aids in separating the fine particles from the large particles at the input, permitting the air to act upon the particles more individually, and reducing the amount of fines otherwise introduced into the receiver sections intended to collect the larger particles. An upward flow of air may also be introduced within the receiver sections, moderated by screens placed above the air inlets, to keep more of the fines entrained and moving toward appropriate receiver sections.
Through the honeycomb and screen section arrangement at the upstream end of the chamber, combined with the drawing of air through the classifier by suction, air turbulence is reduced and, particularly when combined with greater separation of the incoming feed stream through vibration, the present invention makes more accurate classification of particulate matter possible.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.